Abstract: LPD on-line monitoring system is adopted in nuclear power plants to calculate and display the real monitoring parameters such as linear power density (LPD) and hot spot factor, which can present the state of the reactor core accurately and in time, thus to improve the economy and safety of the nuclear power plant. The overall error of LPD should be considered in the alarm limit settings of LPD, with certain margin. The analysis method for overall uncertainty of LPD on-line monitoring in HPR1000 is studied, and combining errors of various parts obtained by statistical method, the overall error limit is obtained. The results show that the error of LPD on-line monitoring system in HPR1000 meets the engineering requirement.